Two familiar examples of invertebrates - earthworm and cockroach are chosen here to study the salient features.
Kingdom : Metazoa
Subkingdom : Eumentazoa
Division : Bilateria
Grade : Eucoelomata
Phylum : Annelida
Class : Oligochaeta
Order : Ophisthopora
Family : Pheritimidae
Genus : Pheretima
Species : posthuma
Pheretima posthuma is a common species of earthworm found living in burrows made in moist soil. It is nocturnal in habit.
It has an elongated, cylindrical body which is metamerically segmented. Anteriorly the first segment is called peristomium. It encloses a mouth. Overhanging the mouth is a fleshy projection called prostomium.
The body wall is dermo muscular. It consists of a thin cuticle, and epidermis and musculature. The cuticle is an elastic non-cellular covering. Epidermis is represented by a layer of columnar epithelial cells, represented by supporting cells, glandular cells, and albumen cells. In addition basal cells are present. The musculature is represented by an outer layer of circular muscles and an inner layer of longitudinal muscles. In between the musculature and coelom there is a thin layer of coelomic epithelium (=peritoneum)
The body cavity is a true coelom since it is lined by a layer of coelomic epithelium of mesodermal origin. The coelom is filled with a watery fluid called coelomic fluid which encloses atleast 4 different types of cells (=corpuscles)
- Phagocytes which can ingest disease producing pathogens.
- Mucocytes which are elongated cells
- Round nucleated cells and
Movement involves musculature of the body wall and structures called setae. Setae are hard horny, chitinous structures embedded in the body wall in small pits. About 1/3 of the length of each setae projects out of the surface of the skin. The setae are arranged in the form of a ring in every segment.
Earthworms feed on dead organic matter, particularly vegetation present in the soil. They also obtain their organic food from the soil which they ingest in the process of their burrowing. Undigested food and soil are eliminated outside as worm castings.
- Chloragogen cells or yellow cells, supposed to be excretory in function
Digestive System of Earthworm
It is of closed type consisting of blood vessels and capillaries which ramify to all parts of the body. The blood is composed of plasma and corpuscles. Plasma is red in colour since it contains hemoglobin in a dissolved state. Corpuscles are colourless.
Excretion is carried out by segmentally arranged coiled ducts called nephridia. Based on their location, these nephridia can be distinguished into three types-
i. Pharyngeal nephridia which occur as paired tufts on either side of the pharynx and oesophagus in the fourth, fifth and sixth segments.
ii. Integumentary nephridia which are scattered on the inner body wall of every segment except the first two. There are about 200 to 250 nephridia in each segment and about 2000 each in the clitellar segments.
iii. Septal nephridia which are found attached to the inter segmental septum of each segment after the 15th segment. The pharyngeal and septal nephridia open into the alimentary canal and hence are known as enteronephric nephridia while the integumentary nephridia open out through nephriodiopores and hence are known as exonephric nephridia.
Nephridial System of an Earthworm
Respiration occurs through the body wall by simple diffusion. Specific respiratory organs are absent.
Nervous system is well developed. It is represented by a nerve ring and a double, ventral nerve cord. The nerve ring is formed by a pair of supra oesophageal ganglia, a pair of suboesophageal ganglia and a pair of circum oesophageal connectives. The nerve cord produces segmental ganglia.
Nervous System of an Earthworm
Reproduction is only by sexual method. Earthworms are bisexual (hermaphrodite). The male reproductive system is represented by two pairs of testes, two pairs of seminal vesicles, two pairs of spermathecal funnels a pair of vasa deferentia (sperm ducts), a pair of prostate glands. The female reproductive system is represented by a pair of ovaries, and oviducal funnel, a pair of oviducts and four pairs of spermathecae.
Fertilisation is cross and external. It occurs following copulation of two mature individuals for exchange of sperms. A cocoon is formed in the clitellar region following copulation and separation of individuals. As the worm wriggles behind, the cocoon is slipped towards posterior end. When the cocoon is on its way it receives ova from the female genital aperture and the sperms collected from another worm, there by allowing cross and external fertilisation.
A cocoon may contain many fertilised eggs. However, only one of them develops at the expense of the others. Development is direct. There is no larval stage. A fully grown young worm crawls out of the cocoon in about two or three weeks. The young worm resembles the adult, except for size and absence of clitellum.